Round Pass Mudflow

The Round Pass Mudflow also underlies parts of Indian Henry's Hunting Ground, which is on the divide between Tahoma Creek and Kautz Creek. The southwest margin of the mudflow ends there in an abrupt front about 17 feet high where it is crossed by the Wonderland Trail about 300 yards south of a patrol cabin. The boundary between the mudflow and the adjacent Evans Creek Drift is conspicuously marked by the absence or presence of pyroclastic layer Y. At Indian Henrys Hunting Ground the mudflow extends to a maximum altitude of about 5,350 feet, which is nearly 1,000 feet higher than the floor of the Tahoma Creek valley directly to the north.

The Round Pass Mudflow crops out at several places along the Tahoma Creek valley. One of these outcrops is a streambank exposure about 1 mile upvalley form Tahoma Creek campground (measured section 9). There the mudflow unconformably overlies coarse fluvial gravel, from which it is separated by pyroclastic layer Y, and is as much as 20 feet thick. The mudflow contains boulders as large as 6 feet in maximum dimension and logs as large as 4 feet in diameter. The mudflow was identified downvalley to a point about 1 miles north of the Nisqually River. At that point it caps a terrace which is about 60 feet higher than the Tahoma Creek flood plain.

The North and South Puyallup River valleys I recognized the mudflow as far downstream as a point 10 miles beyond the park boundary, and it may have extended even farther. Remnants of the mudflow veneer the valley walls of the North Puyallup River up to a maximum height of about 350 feet above the valley floor directly north of Klapatche Park. Farther downstream, a quarter of a mile west of Mount Rainier National Park, the mudflow coast the valley walls to a maximum height of about 240 feet. The mudflow, exposed there in cuts along logging roads, lies on top of layer Y and talus. In these exposures, the base of the pumice layer contains many small pieces of charcoal, but none were noted either at the top of the layer or in the overlying mudflow. About three-quarters of a mile farther downvalley, near the mouth of the North Puyallup River, cuts along another logging road expose the Round Pass Mudflow up to a height of a little more than 200 feet above the river.

In the South Puyallup River valley, remnants of the mudflow coat the valley walls to a height of about 1,000 feet above the river at a point 1.7 miles north of Round Pass, about 350 feet at the mouth of St. Andrews Creek just west of the park, and about 250 feet at the northwest end of Klapatche Ridge. Near the mouth of the Mowich River, 5 1/2 miles farther downstream, the highest remnants are 160 feet higher than the Puyallup River, and the base of the mudflow is below river level. An imaginary line connecting these high remnants of the mudflow has a downvalley slope of about 550 feet per mile between Round Pass and the mouth of St. Andrews Creek, whereas the valley floor now has a gradient of about 300 feet per mile between these points. However, this line might not represent the sloping upper surface of a continuous, deep, flowing stream of mud, but instead could record the maximum height reached by one or more transient waves of mudflow. Below St. Andrews Creek, where the valley widens appreciably, the slope, as reconstructed from high mudflow remnants, decreases and becomes more nearly parallel to the present slope of the valley floor.

The Round Pass Mudflow underlies a broad area in the Puyallup River valley from the mouth of Deer Creek downstream to the mouth of LeDoux Creek. The Puyallup River has cut a trench as much as 100 feet deep into this mudflow fill. Although the Round Pass Mudflow may underlie the floor of the Puyallup River valley beyond the mountain front, I did not recognize it in the outcrops while mapping the valley nor did I identify it in logs of wells beneath the valley floor.

(Section Missing) ...

Because remnants of the Round Pass Mudflow can be found on the slopes of the volcano at least as far as the east end of Emerald Ridge and to an altitude of nearly 7,000 feet on the ridge that extends from St. Andrews Park to Puyallup Cleaver, I presume that the mudflow originated somewhere on the flank of the volcano upslope from these points. The mudflow's clay minerology suggests derivation from masses of altered and partly altered rock similar to those now present in the east wall of Sunset Amphitheater. The mudflow probably originated as a massive avalanche or series of avalanches of this kind of rock. The distribution of the mudflow as a veneer on older deposits in the valleys close to Mount Rainier indicates that the mud was very fluid and that almost all of it drained away after the crest of the mudflow passed downvalley.

The heights reached by the mudflow suggest that the flow had enough volume to fill temporarily the upper South Puyallup and Tahoma Creek valleys to a depth of at least 1,000 feet, either as a flowing stream of mud or as one or more large waves. The mass of the mudflow needed to form a deep flowing stream of mud probably is adequately represented in the Puyallup River drainage by extensive deposits which probably have a volume of at least 200 million cubic yards in the area west of the park. Because of its water content, the lahar must have had a substantially greater volume while moving. Deposits of comparable volume have not been found in the Tahoma Creek valley, and the mudflow apparently had a depth of only a few tens of feet at a point just 4 miles downvalley from Round Pass. These relations may have resulted from movement of the mudflow down Tahoma Creek valley as a single huge transient wave, which progressively decreased in height, similar to that inferred for the Paradise lahar. Substantial amounts of the mudflow were left only in areas where a low slope did not permit it to drain away; elsewhere veneers only a few thick were left on the sides and floor of the valley. Different volumes of the mudflow in the three valleys may have resulted form slightly different directions of initial downslope movement of successive avalanches from Sunset Amphitheater, which caused unequal amounts of debris to move into respective valleys. Or, the different volumes could have been caused by unequal lateral distribution of rock debris within a single enormous avalanche.

Three radiocarbon dates have been determined on wood from logs contained in the Round Pass Mudflow. One of these was from a streambank exposure in the Tahoma Creek Valley; it had a radiocarbon age of 2,610+/-350 years. Two other samples were from streambank outcrops near the mouth of the Mowich River. One sample, from the north bank 500 feet upstream from the river's mouth, had a radiocarbon age of 2,170+/-200 years. The other, form the south bank 500 feet further upstream, had an age of 2,710+/-250 years. When corrected, these three dates are about 2,800, 2.200, and 2,900 years, respectively. The reason for the substantial age difference is not known because there is no stratigraphic evidence in the outcrops near the mouth of the Mowich River of two mudflows. I assume that the Round Pass Mudflow is about 2,800 years old.